Extremely short duration optical pulses, which are also known as femtosecond pulses, are important for high speed signal processing, communications, micro-machining, imaging, sensing applications, time resolved experiments, where short-lived transient species can be observed in biological and chemical reactions.
The early development of laser technology involving the production of extremely short pulses is disclosed in U.S. Pat. No. 4,727,553 to Fork et al. The early version of a passively mode-locked laser is described as containing a saturable absorbing element optically coupled to a gain medium in an optical resonator. Additional variations of the saturable absorbing element positioned between reflective (mirror-like) elements are disclosed in U.S. Pat. No. 5,237,577 to Keller et al. and U.S. Pat. No. 5,278,855 to Jacobovitz-Veselka et al. The so-called Semiconductor Saturable Absorber Mirrors (SESAMs) are described in U.S. Pat. Nos. 6,538,298 B1 and 6,393,035 B1 to Weingarten et al. The saturable absorber element functions as a shutter.
Further development of passively mode-locked lasers includes use of astigmatic mirrors with spacing and a unique twist angle to correct the optical path in an absorption cell (U.S. Pat. No. 5,291,265 to Kebabian) and subsequently, the integration of the saturable absorber with the optical element as discussed below.
Current conventional, commercial, ultrafast mode-locked lasers have basic constituents, which include an active laser medium, resonator mirrors, and optical components, usually prisms that compensate for dispersion in the resonator. The mode-locking element in simpler devices is a nonlinear optical effect occurring in the laser medium itself. A typical mode-locked laser design is shown in FIG. 1; the laser of this design has drawbacks in that it is not “self-starting” and is sensitive to effects of alignment, optical pumping, and the like.
More recently, other components have been added to the general structure shown in FIG. 1. Instead of using prisms as the dispersive elements, special mirrors, known as, Chirped Mirrors have been developed. Chirped Mirrors or Negative Group Velocity Dispersion (NGVD) as discussed in U.S. Pat. No. 6,055,261 to Reed et al. have been used to provide an ultrafast laser device with a significantly shortened resonant cavity. Additional prism replacements include a fold mirror (U.S. Pat. No. 5,812,308 to Kafka et al.); a dispersive dielectric mirror (U.S. Pat. No. 5,734,503 to Szipocs et al.); a self-tuning saturable reflector comprising two Bragg reflectors (U.S. Pat. No. 6,141,359 to Cunningham et al.); and heated mirrors (U.S. Pat. No. 6,188,475 to Inman et al.) in semiconductor processing.
The prior art includes several arrangements of gain elements, optical components, resonator mirrors, and mode-locking elements for short pulse lasers; however, all arrangements are unlike the arrangements of elements in the present invention. FIG. 1 shows the basic architecture for a short pulse laser, a high reflector (HR) mirror 10, Kerr-lens lasing element 20, dispersion compensating elements, such as, a prism pair 30 and an output coupler (OC) 40. In such lasers, the Kerr-lens effect in the gain medium is the mode-locking mechanism. This configuration is a simpler device that is not “self-starting” and is sensitive to effects of alignment, optical pumping, and the like. FIG. 2 shows the substitution of a saturable absorber mirror 50 for the Kerr-lens mirror 20 (in FIG. 1), a lasing element 60 and Chirped mirrors 70, as the dispersion compensating element, with output coupler 80 to provide a laser that is more stable and “self-starting.”
FIG. 3 is another prior art arrangement of a mode-locked laser with a high reflector (HR) 90, a multipass cell 92, a Kerr-lens lasing element 94, a prism pair 96 as the dispersion compensating element with output coupler 98. The multipass cell 92 is added to slow down the repetition rate.
None of the prior art arrangements of gain elements, or optical elements have the addition of Chirped Mirrors (CP), multi-pass mirror system and Saturable Absorber Mirror (SAM) mode-locking elements as disclosed herein. A second embodiment of the invention includes a cavity-dumping feature in the novel arrangement of elements. The cavity-dumping feature facilitates the extraction of all energy trapped inside the cavity by dumping the beam and thereby providing a several-fold improvement in the usable pulse energy. The present invention has a unique configuration and meets the commercial need for rugged, low cost, high power, ultra-short pulse lasers useful in, but not limited to, micro-processing and micro-structuring below conventional tolerances.